In the United States and many other countries much goods is transported over highways in trailers, or more particularly semi-trailers. The movement of goods by highway trailer is quite efficient for short hauls because of its nearly unlimited flexibility as to origin and destination of shipments. However, long hauls between major metropolitan and manufacturing centers generally involve no advantage in flexibility but do have relatively high fuel, labor and equipment costs. To overcome these disadvantages, railroads have provided piggy-back transportation in which trailers are carried on railroad flat cars. This method of shipping is rapid, relatively low cost, fuel efficient, involves no wear on the trailer and eliminates any need to unload the trailer lading between the place of origin and the destination. The railroads have recognized for some time, however, that if the flat car could be eliminated and trains formed solely by coupling the trailers together end-to-end that far less weight would have to be moved over the track. This would lower fuel consumption, lower terminal time and expense, reduce capital investment by eliminating a need for trailer carrying flat cars and would reduce railroad track wear and road bed repairs.
One system previously proposed for making a train of highway trailers is to support the back or rear end of each trailer on a railroad truck of the four-wheel two-axle type. The front end of each trailer, all of which face in the same direction, is then connected to the truck or to the back end of the adjoining forward trailer. Each railroad truck is releasably connected to the trailer it supports by means of an adapter. One train embodiment of this type is disclosed in Dobson U.S. Pat. No. 2,963,986. The adapter disclosed in the reference, however, is not considered to be structurally acceptable so that an improved adapter for the stated purpose is highly desirable.
A train of highway trailers is formed by delivering the trailers, loaded or empty, to a railroad freight yard where an inventory of dedicated and suitably adapted railroad trucks is stored on the tracks. The rear end of a trailer is then positioned above the truck, the over the highway wheels on the trailer are then lifted, as by air springs, and the trailer lowered or backed up so that means on the trailer can engage with means on the truck to releasably connect them together. Then the front end of another trailer is connected to the truck or to the rear of the other trailer. In this manner, the rear of one trailer and the front of another trailer are supported by a single truck. A plurality of trailers are coupled together as described to make a train of any desired length.
A train of trailers formed as described has characteristics which distinguish it from a conventional freight train. For example, the cars of a freight train, whether they be coal cars, box cars, flat cars, gondola cars, tank cars, auto rack cars or other types of cars, are much heavier than highway trailers. Additionally, railroad cars are conventionally supported at each end by individual railroad trucks, which in most cases are of the four-wheel two-axle type, although to a more limited extent of the two-wheel single-axle type. Also, multiunit articulated cars have been built in which the ends of adjacent units rest on and share a common truck.
Because of the structural differences, the dynamics involved in a train made of such railroad cars, and a train of highway trailers, are substantially different so that what is suitable for one train is not automatically suitable for the other. Thus, in designing for trains of trailers it has been considered necessary to strengthen the trailers so that they can withstand the rocking and twisting forces, and longitudinal train action forces, to which they are subjected in rail travel. Although the trailers have been strengthened they have substantially less strength than a conventional freight car and therefore the suspension system must be improved to insure the trailer's structural adequacy against fatigue failure.
Since the loads applied by the trailers to the trucks are lighter than the loads applied by conventional railroad cars the most widely used least expensive trucks were selected for use in trains of trailers. Trains formed of such trucks, however, have been found to provide far less than desirable performance, especially at speeds above forty-five miles per hour, because of excessive lateral instability due to truck hunting. Truck hunting causes the rolling wheels and axle assembly of a standard railway freight car truck to move along a pair of rails in a sinusoidal pattern. A need accordingly exists for a train of trailers which can travel at increased speeds with lower lateral instability due to truck hunting.